Ghost cancelling systems nominally consist of two parts, a programmable filter and circuitry for calculating a model of the signal transmission channel. The channel modelling circuitry is responsive to a transmitted reference signal and a stored idealized reference signal for determining the characteristics of the transmission channel (in particular characteristics which may give rise to multipath distortion). From such characteristics, the channel modelling circuitry generates coefficients for programming the programmable filter to pass video signal with multipath distortion substantially eliminated. For a more detailed description of channel modelling techniques see U.S. Pat. No. 4,864,403, issued Sep. 5, 1989 and titled "Adaptive Television Ghost Cancellation System Including Filter Circuitry with Non-Integer Sample Delay" or GHOST REDUCTION BY REPRODUCTION by N. Komlya, IEEE Transactions on Consumer Electronics, August 1992, Vol. 38, No. 3, pp. 195-199, for example. The programmable filters are typically of two types, finite impulse response (FIR) and infinite impulse response (IIR). Some ghost cancelling systems employ FIR filters, others employ IRR filters, but the majority of systems use a combination of both.
Both the IIR and FIR ghost cancelling filters employ tapped delay lines providing a plurality of relatively delayed signals. The delayed signals are weighted by programmable coefficients, and the weighted coefficients are combined to provide a filtered output signal. The FIR and IIR filters have relative advantages and disadvantages but it is well known that FIR filters are inherently stable, and that IIR filters give rise to stability problems. The present invention is directed toward ameliorating instabilities in ghost cancelling systems which use IIR type ghost cancelling filters.